Ip Address Sub Netting Tutorial

IP Address Subnetting Tutorial Index: •

Introduction

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IP Addressing

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Subnetting

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More Restrictive Subnet Masks

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An Example

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CIDR

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Allowed Class A Subnets

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Allowed Class B Subnets

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Allowed Class C Subnets

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Logical Operations

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Internet Sources and References

Introduction •

What is an IP Address?

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What are Classes?

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What is a Network Address?

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What are Subnet Masks and Subnet Addresses?

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How are Subnet Masks defined and used?

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How can all this be applied?

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What is CIDR?

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How can I get more information?

Note: This tutorial is designed to be viewed on line. I have received numerous requests to have a text-only or single-page printable version of the Tutorial. No such document exists. If you need a hard copy of this Tutorial, you can print each page in turn with your browser's print function. I hope this is sufficient for most people.

IP Addressing An IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network. An IP address is a 32 bit binary number usually represented as 4

decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal" notation.

Example: 140.179.220.200 It is sometimes useful to view the values in their binary form. 140 .179 .220 .200 10001100.10110011.11011100.11001000

Every IP address consists of two parts, one identifying the network and one identifying the node. The Class of the address and the subnet mask determine which part belongs to the network address and which part belongs to the node address.

Address Classes There are 5 different address classes. You can determine which class any IP address is in by examining the first 4 bits of the IP address. •

Class A addresses begin with 0xxx, or 1 to 126 decimal.

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Class B addresses begin with 10xx, or 128 to 191 decimal.

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Class C addresses begin with 110x, or 192 to 223 decimal.

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Class D addresses begin with 1110, or 224 to 239 decimal.

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Class E addresses begin with 1111, or 240 to 254 decimal.

Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine; [You can test this: you should always be able to ping 127.0.0.1, which points to yourself] Class D addresses are reserved for multicasting; Class E addresses are reserved for future use. They should not be used for host addresses. Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N, in blue) and which part belongs to the node (n, in red). •

Class A -- NNNNNNNN.nnnnnnnn.nnnnnnnn.nnnnnnnn

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Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn

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Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn

In the example, 140.179.220.200 is a Class B address so by default the Network part of the address (also known as the Network Address) is defined by the first two octets (140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200). In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, 140.179.0.0 specifies the network address for 140.179.220.200. When the node section is set to all "1"s, it specifies a broadcast that is sent to all hosts on the network. 140.179.255.255 specifies the example broadcast address. Note that this is true regardless of the length of the node section. Private Subnets There are three IP network addresses reserved for private networks. The addresses are 10.0.0.0, Subnet Mask 255.0.0.0, 172.16.0.0, Subnet Mask 255.240.0.0, and 192.168.0.0, Subnet Mask 255.255.0.0. These addresses are also notated 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16; this notation will be explained later in this tutorial. They can be used by anyone setting up internal IP networks,

such as a lab or home LAN behind a NAT or proxy server or a router. It is always safe to use these because routers on the Internet by default will never forward packets coming from these addresses. These addresses are defined in RFC 1918.

Subnetting Subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used to connect IP networks to minimize the amount of traffic each segment must receive. Subnet Masking

Applying a subnet mask to an IP address allows you to identify the network and node parts of the address. The network bits are represented by the 1s in the mask, and the node bits are represented by the 0s. Performing a bitwise logical AND operation between the IP address and the subnet mask results in the Network Address or Number. For example, using our test IP address and the default Class B subnet mask, we get: 10001100.10110011.11110000.11001000 140.179.240.200 11111111.11111111.00000000.00000000 255.255.000.000 Subnet Mask -------------------------------------------------------10001100.10110011.00000000.00000000 140.179.000.000

Class B IP Address Default Class B Network Address

Default subnet masks: •

Class A - 255.0.0.0 - 11111111.00000000.00000000.00000000

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Class B - 255.255.0.0 - 11111111.11111111.00000000.00000000

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Class C - 255.255.255.0 - 11111111.11111111.11111111.00000000

More Restrictive Subnet Masks Additional bits can be added to the default subnet mask for a given Class to further subnet, or break down, a network. When a bitwise logical AND operation is performed between the subnet mask and IP address, the result defines the Subnet Address (also called the Network Address or Network Number). There are some restrictions on the subnet address. Node addresses of all "0"s and all "1"s are reserved for specifying the local network (when a host does not know its network address) and all hosts on the network (broadcast address), respectively. This also applies to subnets. A subnet address cannot be all "0"s or all "1"s. This also implies that a 1 bit subnet mask is not allowed. This restriction is required because older standards enforced this restriction. Recent standards that allow use of these subnets have superseded these standards, but many "legacy" devices do not support the newer standards. If you are operating in a controlled environment, such as a lab, you can safely use these restricted subnets. To calculate the number of subnets or nodes, use the formula (2n-2) where n = number of bits in either field, and 2n represents 2 raised to the nth power. Multiplying the number of subnets by the

number of nodes available per subnet gives you the total number of nodes available for your class and subnet mask. Also, note that although subnet masks with non-contiguous mask bits are allowed, they are not recommended. Example: 10001100.10110011.11011100.11001000 140.179.220.200 11111111.11111111.11100000.00000000 255.255.224.000 -------------------------------------------------------10001100.10110011.11000000.00000000 140.179.192.000 10001100.10110011.11011111.11111111 140.179.223.255

IP Address Subnet Mask Subnet Address Broadcast Address

In this example a 3 bit subnet mask was used. There are 6 (23-2) subnets available with this size mask (remember that subnets with all 0's and all 1's are not allowed). Each subnet has 8190 (2132) nodes. Each subnet can have nodes assigned to any address between the Subnet address and the Broadcast address. This gives a total of 49,140 nodes for the entire class B address subnetted this way. Notice that this is less than the 65,534 nodes an unsubnetted class B address would have. You can calculate the Subnet Address by performing a bitwise logical AND operation between the IP address and the subnet mask, then setting all the host bits to 0s. Similarly, you can calculate the Broadcast Address for a subnet by performing the same logical AND between the IP address and the subnet mask, then setting all the host bits to 1s. That is how these numbers are derived in the example above. Subnetting always reduces the number of possible nodes for a given network. There are complete subnet tables available here for Class A, Class B and Class C. These tables list all the possible subnet masks for each class, along with calculations of the number of networks, nodes and total hosts for each subnet.

An Example Here is another, more detailed, example. Say you are assigned a Class C network number of 200.133.175.0 (apologies to anyone who may actually own this domain address). You want to utilize this network across multiple small groups within an organization. You can do this by subnetting that network with a subnet address. We will break this network into 14 subnets of 14 nodes each. This will limit us to 196 nodes on the network instead of the 254 we would have without subnetting, but gives us the advantages of traffic isolation and security. To accomplish this, we need to use a subnet mask 4 bits long. Recall that the default Class C subnet mask is 255.255.255.0 (11111111.11111111.11111111.00000000 binary)

Extending this by 4 bits yields a mask of 255.255.255.240 (11111111.11111111.11111111.11110000 binary)

This gives us 16 possible network numbers, 2 of which cannot be used: Subnet bits Network Number Node Addresses Broadcast Address 0000 200.133.175.0 Reserved None 0001 200.133.175.16 .17 thru .30 200.133.175.31 0010 200.133.175.32 .33 thru .46 200.133.175.47 0011 200.133.175.48 .49 thru .62 200.133.175.63 0100 200.133.175.64 .65 thru .78 200.133.175.79

0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

200.133.175.80 200.133.175.96 200.133.175.112 200.133.175.128 200.133.175.144 200.133.175.160 200.133.175.176 200.133.175.192 200.133.175.208 200.133.175.224 200.133.175.240

.81 thru .94 .97 thru .110 .113 thru .126 .129 thru .142 .145 thru .158 .161 thru .174 .177 thru .190 .193 thru .206 .209 thru .222 .225 thru .238 Reserved

200.133.175.95 200.133.175.111 200.133.175.127 200.133.175.143 200.133.175.159 200.133.175.175 200.133.175.191 200.133.175.207 200.133.175.223 200.133.175.239 None

CIDR -- Classless InterDomain Routing Now that you understand "classful" IP Subnetting principals, you can forget them ;). The reason is CIDR -- Classless InterDomain Routing. CIDR was invented several years ago to keep the internet from running out of IP addresses. The "classful" system of allocating IP addresses can be very wasteful; anyone who could reasonably show a need for more that 254 host addresses was given a Class B address block of 65533 host addresses. Even more wasteful were companies and organizations that were allocated Class A address blocks, which contain over 16 Million host addresses! Only a tiny percentage of the allocated Class A and Class B address space has ever been actually assigned to a host computer on the Internet. People realized that addresses could be conserved if the class system was eliminated. By accurately allocating only the amount of address space that was actually needed, the address space crisis could be avoided for many years. This was first proposed in 1992 as a scheme called Supernetting. Under supernetting, the classful subnet masks are extended so that a network address and subnet mask could, for example, specify multiple Class C subnets with one address. For example, If I needed about 1000 addresses, I could supernet 4 Class C networks together: 192.60.128.0 (11000000.00111100.10000000.00000000) Class C subnet address 192.60.129.0 (11000000.00111100.10000001.00000000) Class C subnet address 192.60.130.0 (11000000.00111100.10000010.00000000) Class C subnet address 192.60.131.0 (11000000.00111100.10000011.00000000) Class C subnet address -------------------------------------------------------192.60.128.0 (11000000.00111100.10000000.00000000) Supernetted Subnet address 255.255.252.0 (11111111.11111111.11111100.00000000) Subnet Mask 192.60.131.255 (11000000.00111100.10000011.11111111) Broadcast address

In this example, the subnet 192.60.128.0 includes all the addresses from 192.60.128.0 to 192.60.131.255. As you can see in the binary representation of the subnet mask, the Network portion of the address is 22 bits long, and the host portion is 10 bits long. Under CIDR, the subnet mask notation is reduced to a simplified shorthand. Instead of spelling out the bits of the subnet mask, it is simply listed as the number of 1s bits that start the mask. In the above example, instead of writing the address and subnet mask as 192.60.128.0, Subnet Mask 255.255.252.0

the network address would be written simply as: 192.60.128.0/22

which indicates starting address of the network, and number of 1s bits (22) in the network portion of the address. If you look at the subnet mask in binary (11111111.11111111.11111100.00000000), you can easily see how this notation works. The use of a CIDR notated address is the same as for a Classful address. Classful addresses can easily be written in CIDR notation (Class A = /8, Class B = /16, and Class C = /24) It is currently almost impossible for an individual or company to be allocated their own IP address blocks. You will simply be told to get them from your ISP. The reason for this is the ever-growing size of the internet routing table. Just 10 years ago, there were less than 5000 network routes in the entire Internet. Today, there are over 100,000. Using CIDR, the biggest ISPs are allocated large chunks of address space (usually with a subnet mask of /19 or even smaller); the ISP's customers (often other, smaller ISPs) are then allocated networks from the big ISP's pool. That way, all the big ISP's customers (and their customers, and so on) are accessible via 1 network route on the Internet. But I digress. It is expected that CIDR will keep the Internet happily in IP addresses for the next few years at least. After that, IPv6, with 128 bit addresses, will be needed. Under IPv6, even sloppy address allocation would comfortably allow a billion unique IP addresses for every person on earth! The complete and gory details of CIDR are documented in RFC1519, which was released in September of 1993.

Allowed Class A Subnet and Host IP addresses # bits 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Subnet Mask 255.192.0.0 255.224.0.0 255.240.0.0 255.248.0.0 255.252.0.0 255.254.0.0 255.255.0.0 255.255.128.0 255.255.192.0 255.255.224.0 255.255.240.0 255.255.248.0 255.255.252.0 255.255.254.0 255.255.255.0 255.255.255.128 255.255.255.192 255.255.255.224 255.255.255.240 255.255.255.248

CIDR /10 /11 /12 /13 /14 /15 /16 /17 /18 /19 /20 /21 /22 /23 /24 /25 /26 /27 /28 /29

# Subnets 2 6 14 30 62 126 254 510 1022 2046 4094 8190 16382 32766 65534 131070 262142 524286 1048574 2097150

# Hosts 4194302 2097150 1048574 524286 262142 131070 65534 32766 16382 8190 4094 2046 1022 510 254 126 62 30 14 6

Nets * Hosts 8388604 12582900 14680036 15728580 16252804 16514820 16645636 16710660 16742404 16756740 16760836 16756740 16742404 16710660 16645636 16514820 16252804 15728580 14680036 12582900

22

255.255.255.252 /30

4194302

2

8388604

Allowed Class B Subnet and Host IP addresses # bits 2 3 4 5 6 7 8 9 10 11 12 13 14

Subnet Mask 255.255.192.0 255.255.224.0 255.255.240.0 255.255.248.0 255.255.252.0 255.255.254.0 255.255.255.0 255.255.255.128 255.255.255.192 255.255.255.224 255.255.255.240 255.255.255.248 255.255.255.252

CIDR /18 /19 /20 /21 /22 /23 /24 /25 /26 /27 /28 /29 /30

# Subnets 2 6 14 30 62 126 254 510 1022 2046 4094 8190 16382

# Hosts 16382 8190 4094 2046 1022 510 254 126 62 30 14 6 2

Nets * Hosts 32764 49140 57316 61380 63364 64260 64516 64260 63364 61380 57316 49140 32764

Allowed Class C Subnet and Host IP addresses # bits 2 3 4 5 6

Subnet Mask 255.255.255.192 255.255.255.224 255.255.255.240 255.255.255.248 255.255.255.252

CIDR /26 /27 /28 /29 /30

# Subnets 2 6 14 30 62

# Hosts 62 30 14 6 2

Nets * Hosts 124 180 196 180 124

Logical Operations This page will provide a brief review and explanation of the common logical bitwise operations AND, OR, XOR (Exclusive OR) and NOT. Logical operations are performed between two data bits (except for NOT). Bits can be either "1" or "0", and these operations are essential to performing digital math operations. In the "truth tables" below, the input bits are in bold, and the results are plain.

AND The logical AND operation compares 2 bits and if they are both "1", then the result is "1", otherwise, the result is "0". 01 000

101

OR The logical OR operation compares 2 bits and if either or both bits are "1", then the result is "1", otherwise, the result is "0". 01 001 111

XOR The logical XOR (Exclusive OR) operation compares 2 bits and if exactly one of them is "1" (i.e., if they are different values), then the result is "1"; otherwise (if the bits are the same), the result is "0". 01 001 110

NOT The logical NOT operation simply changes the value of a single bit. If it is a "1", the result is "0"; if it is a "0", the result is "1". Note that this operation is different in that instead of comparing two bits, it is acting on a single bit. 01 1 0